Primary bone tumors confront the surgeon with the problem of not only eradication of the lesion but also restoration of normal extremity function. Though amputation sometimes is necessary, a wide variety of osseous lesions are amendable by radical en bloc resection and prosthetic bone and joint replacement. The functional requirements for such replacement are more stringent than for standard joint replacement. Many of the related problems are of a biomechanical nature and have to be studied under the close collaboration of many disciplines. The objectives of this project are: 1) to establish proper design guidelines for such custom implants, 2) to apply bone ingrowth materials to the prosthesis to obtain more secure fixation without the use of bone cement, 3) to analyze the change in joint function due to soft tissue alteration, and 4) to quantitate functional deficits for the improvement of rehabilitative programs for postoperative care. The anatomical regions include the hip and proximal femur, the knee with either the distal femur or proximal tibia, the proximal humerus, and any segment of long bone away from the joint. Stress analysis based on theoretical and experimental methods will be used to ensure the necessary strength requirement in the prosthesis-bone system. Objective gait analysis and joint strength tests will be utilized to assess the functional performance of the patients after custom tumor prosthetic replacement. The results of this study should provide an effective treatment alternative for primary bone tumor patients. Such a method can also be used on metastatic cancer patients with bone lesions to extend their functional life and reduce nursing problems. Experience gained here would assist in the management of patients with failed regular total joints which must be salvaged due to substantial bone loss.